Rovibrationally selected and resolved pulsed field ionization-photoelectron study of propyne: Ionization energy and spin-orbit interaction in propyne cation

By using a high-resolution infrared (IR) laser to prepare propyne $(C3H4)$ in selected rotational levels of the excited $ν1$ (acetylenic C–H stretching) vibration mode prior to vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have obtained rotationally resolved VUV-PFI-PE spectra for the $C3H4+(X̃E3∕2,1∕22,ν1+=1)$ band. The analysis of these PFI-PE spectra leads to the determination of the spin-orbit constant of $A=−13.0±0.2cm−1$ for the $C3H4+(X̃E3∕2,1∕22,ν1+=1)$ state. Using this $A$ constant and the relative rotationally selected and resolved state-to-state photoionization cross sections thus measured, we have obtained an excellent simulation for the VUV-PFI-PE origin band of $C3H4+(X̃E3∕2,1∕22)$⁠, yielding a value of $83619.0±1.0cm−1$ $(10.36744±0.00012eV)$ for the adiabatic ionization energy of $C3H4$ $[IE(C3H4)]$⁠. The present two-color IR-VUV-PFI-PE study has also made possible the determination of the C–H stretching frequencies $ν1+=3217.1±0.2cm−1$ for $C3H4+(X̃E3∕2,1∕22)$⁠. The spectral assignment and simulation were guided by high-level ab initio calculations on the $IE(C3H4)$⁠, Franck-Condon factors for photoionization transitions, and rotational constants and vibrational frequencies for $C3H4+$⁠.